The invention provides methods and compositions through genetic engineering wherein a transgenic alfalfa plant has reduced levels of acid detergent lignin (ADL), reduced levels of guaiacyl lignin (G lignin), improved levels of neutral detergent fiber digestibility (NDFD), and plant vigor comparable to the wild type, resulting in improved agronomic fitness. Forage crops, including legumes, grasses, corn silage, and brassicas, among others, are grown throughout the world to provide animal feed with digestible fiber. Alfalfa and/or alfalfa grass hay is the most important hay crop in the United States. Although predominantly fed as baled hay, it can also be fed as silage, chopped hay, cubes, or pellets. Alfalfa (Medicago sativa) is a forage legume and may comprise 23% to 34% of dairy cattle feed. Quality alfalfa hay is highly digestible and provides high protein, energy, vitamins and minerals. Quality alfalfa also contains less celluloses and hemicelluloses, less lignin(s), lower fiber, and higher relative feed value. Alfalfa feed value is often measured as NDFD. Analyzing alfalfa forage for NDFD provides an estimation of the energy a cow is able to obtain from that forage, and there is a need to the enhance fiber digestibility in alfalfa. For example, an increase in one percentage unit NDFD results in a 0.37-pound increase in forage dry matter intake per day (lb/d), and a 0.55 lb/d increase in 4% fat corrected milk (FCM) yield. Cows fed forages with greater NDFD are able to obtain more total energy and nutritive value from the forages. As a result, the energy requirements can be fulfilled with less grain provided in the diet. Lower NDFD in forage legumes is most often related to the maturity of the forage plant, which is accompanied by an increase in lignin concentrations and associated with an increase in cellulose fibers.
Factors that affect NDFD include the amount of acid detergent fiber (ADF). ADF refers to the cell wall portions of the forage, which is the most indigestible part of forage and includes lignin, cellulose, silica, and insoluble forms of nitrogen. These values are particularly important because they relate to the digestibility of the forage by livestock. Forages with higher ADF are lower in digestible energy than forages with lower ADF. Thus, as the ADF level increases, the digestible energy levels decrease.